Fluorescent compositions, x-ray intensifying screens, and processes for making same

ABSTRACT

An x-ray intensifying screen comprises a support having thereon a fluorescent composition comprising: 
     (a) from 50 to 90 percent by weight of a substantially isotropic phosphor which is excited by x-rays and substantially transparent to light emitted by said phosphor; and 
     (b) from 10 to 50 percent by weight of a polymer having an index of refraction within 0.02 of the index of refraction of said phosphor over at least 80 percent of the emission spectrum of said phosphor, said support having an index of refraction up to or equal to 0.05 units higher than the index of refraction of said phosphor and having a reflection optical density of at least 1.7 to light emitted by said phosphor. 
     The screen is highly transparent, and further exhibits improved contrast and resolution. A preferred fluorescent composition useful in the x-ray intensifying screen comprises: 
     (a) from 50 to 90 percent by weight of a substantially isotropic phosphor which is excited by x-rays and substantially transparent to light emitted by said phosphor; and 
     (b) from 10 to 50 percent by weight of a polymer having an index of refraction with 0.02 of the index of refraction of said phosphor over at least 80 percent of the emission spectrum of said phosphor, said polymer comprising: 
     (i) from 5 to 99 mole percent of recurring units having the formula: ##STR1##  wherein: R 1  is H or alkyl; and 
     R 2  is alkyl, cycloalkyl, aryl, aralkyl or aryl substituted with alkyl, alkoxy or heterocyclic; and 
     (ii) from 1 to 95 mole percent of recurring units having the formula: ##STR2##  wherein: Ar is arylene; 
     R 1  is H or alkyl; 
     R 3  is H, alkyl, aryl, or aralkyl; and 
     R 4  is H, alkyl, alkoxy, amino, halogen, sulfide, sulfoxide, sulfonate or heterocyclic.

This is a continuation of application Ser. No. 238,404, filed Feb. 26,1981, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to transparent x-ray intensifying screens andprocesses for making x-ray intensifying screens for use in radiography,and to fluorescent compositions comprising an isotropic phosphortransparent to x-rays and a polymeric binder.

2. Description Relative to the Prior Art

Transparent x-ray screens comprising alkali halide, alkaline earthhalide, metal sulfide, and metal selenide phosphors have been preparedby various methods. These transparent screens have been shown to bedesirable, because they make more efficient use of impinging x-rayradiation than thick conventional scattering screens, which "waste" amaterial amount of the radiation in diffusion of the light emitted nearthe back of the screen and internal absorption. Thick transparentscreens, having a decreased number of reflections permit this light toreach the front surface of the screen with minimal deflection and toform a sharper image on the photographic film in contact with thescreen. A greater proportion of the x-ray energy, absorbed by thephosphor and converted to light, is utilized in producing images withoutloss of sharpness.

Thin transparent screens, prepared by vapordeposition and containingonly a phosphor, have also been made and exhibit lower speeds thanscattering screens with equal phosphor coverage. Further, lacking aprotective binder, these transparent screens are fragile and highlysusceptible to physical damage. Thicker screens have been made by hotpressing but other defects in the manufacture of these large platesrender them expensive to prepare.

U.S. Pat. No. 3,023,313, issued February 27, 1962 to De La Mater et al.discloses the use of a polymeric binder with a refractive index as closeto that of an alkali metal halide phosphor as possible in order toproduce x-ray intensifying screens with improved speed. However, becauseof substantial differences between the refractive index of selectedbinders and the refractive index of the phosphor, reflecting pigmentsmust be added to the mixture to prevent "blurring of the image" andimprove resolution. Thus, these screens are not truly transparent tolight, and some decrease in utilization of absorbed x-rays is observed.The screens of De La Mater comprise a support preferably having a highlyreflective base coating.

Swank, Applied Optics, 12, 1865-1870 (1973) describes the theoreticalcalculation of modulation transfer function (MTF), related to resolvingpower, of x-ray intensifying screens comprising transparent phosphorsand a black backing. Swank discloses that although the MTF is enhancedwhen a black backing is used, 50% of the exposing radiation is absorbedby the backing. Thus, the speed of the x-ray intensifying screen isreduced.

Gasper, J. Opt. Soc. Am., 63, 714-720 (1973) describes the computationof theoretical efficiencies and MTFs of various screen-receiver systems,and reports that if a dark antihalation undercoat is applied to the backsurface of a transparent screen, the MTF is only slightly improved. If,on the other hand, the back surface is made perfectly reflecting, thereis degradation of MTF, but the efficiency of the screen isadvantageously doubled, as is shown in FIG. 8 of Gasper.

Experimental verification of the Gasper calculations is provided bymeasuring the MTF of a transparent hot-pressed zinc sulfide screencoated with a dyed gelatin undercoat. Excellent agreement was foundbetween the measured and computed MTFs. Gasper concludes that attemptsto improve the MTF of a transparent screen result in an undesirable lossof efficiency. Given a choice between slight increases in MTF coupledwith undesirable losses in efficiency (with an absorbing undercoat), andgreat increases in efficiency coupled with only slightly lower MTFs(reflective undercoat), the high efficiency screen with a reflectiveundercoat is clearly preferred by Gasper.

It is seen that transparent x-ray intensifying screens providing highresolution, while maintaining speed and efficiency, and which areresistant to physical damage and are easily and economicallymanufactured, are extremely desirable.

SUMMARY OF THE INVENTION

An x-ray intensifying screen according to this invention comprises asupport having thereon a fluorescent composition comprising:

(a) from 50 to 90 percent by weight of a substantially isotropicphosphor which is excited by x-rays and substantially transparent tolight emitted by said phosphor; and

(b) from 10 to 50 percent by weight of a polymer having an index ofrefraction within 0.02 of the index of refraction of said phosphor overat least 80 percent of the emission spectrum of said phosphor;

said support having an index of refraction equal to or up to 0.05 unitshigher than the index of refraction of said phosphor and having areflection optical density of at least 1.7 to light emitted by saidphosphor. Using this x-ray intensifying screen, high resolution and highcontrast are obtained, while maintaining high speed, efficiency andresistance to physical damage. Further, the screens can be easilymanufactured and do not require the addition of reflection pigments toprevent image blurring.

It has also been found that a particularly advantageous fluorescentcomposition comprises:

(a) from 50 to 90 percent by weight of a substantially isotropicphosphor which is excited by x-rays and substantially transparent tolight emitted by said phosphor; and

(b) from 10 to 50 percent by weight of a polymer having an index ofrefraction within 0.02 of the index of refraction of said phosphor overat least 80 percent of the emission spectrum of said phosphor, saidpolymer comprising:

(i) from 5 to 99 mole percent of recurring units having the formula:##STR3## wherein: R¹ is H or alkyl; and

R² is alkyl, cycloalkyl, aryl, aralkyl or aryl substituted with alkyl,alkoxy, or heterocyclic; and

(ii) from 1 to 95 mole percent of recurring units having the formula:##STR4## wherein: Ar is arylene;

R¹ is H or alkyl;

R³ is H, alkyl, aryl, or aralkyl; and

R⁴ is H, alkyl, alkoxy, amino, halogen, sulfide, sulfoxide, sulfonate orheterocyclic.

In a further embodiment of the invention, a process for making an x-rayintensifying screen comprises the steps of:

(a) coating a mixture comprising:

(i) from 50 to 90 percent by weight of a substantially isotropicphosphor which is excited by x-rays and substantially transparent tolight emitted by said phosphor; and

(ii) from 10 to 50 percent by weight of at least one copolymerizablemonomer or mixture of monomers, said monomer or mixture of monomers,when polymerized, having an index of refraction within 0.02 of the indexof refraction of said phosphor over at least 80 percent the emissionspectrum of said phosphor,

on a support having an index of refraction equal to or up to 0.05 unitshigher than the index of refraction of said phosphor and having areflection optical density of at least 1.7 to light emitted by saidphosphor; and

(b) polymerizing said mixture coated on said support to produce apolymer comprising recurring units of said monomer or monomer mixture.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A novel x-ray intensifying screen comprises a support having thereon afluorescent composition comprising):

(a) from 50 to 90 percent by weight of a substantially isotropicphosphor which is excited by x-rays and substantially transparent tolight emitted by said phosphor; and

(b) from 10 to 50 percent by weight of a polymer having an index ofrefraction within 0.02 of the index of refraction of said phosphor overat least 80 percent of the emission spectrum of said phosphor;

said support having an index of refraction equal to or up to 0.05 unitshigher than the index of refraction of said phosphor and having areflection optical density of at least 1.7 to light emitted by saidphosphor.

Any substantially isotropic phosphor which is excited by x-rays andsubstantially transparent to the light emitted by the excited phosphoris useful in preparing the fluorescent composition. The term"substantially isotropic phosphor" is used herein to mean a crystallinephosphor having substantially the same optical properties in alldirections of the crystal, and that the crystalline phosphor issubstantially free from defects, such as cracks and inclusions, whichcause scattering of light. Useful phosphors include activated alkalimetal halides, such as KCl:Sb, CsBr:Tl, KI:Tl, KBr:TI, KCl:TI, RbCl:Tl,RbBr:Tl and RbI:Tl; alkaline earth halides such as BaF₂ and BaFCl;activated alkaline earth halides such as CaF₂ : Eu, SrCl₂ : Sm, SrF₂ :Eu, BaFCl:Sr, Eu, BaFCl:Eu and SrF₂ Sm; activated metal silicates suchas BaSiO₃ : Eu, CaSiO₃ :Mn and Zn₂ SiO₄ : Mn; mixed metal fluorides suchas KCdF₃ :Mn and CsCdF₃ :Mn; metal sulfates such as lanthanide-activatedmetal sulfates such as BaSO₄ :Sr, Eu, SrSO₄ :Eu, BaSO₄ :Eu, ZnSO₄ :Mnand Cs₃ SO₄ :Ce; metal gallates such as ZnGa₂ O₄ :Mn; and phosphatessuch as lanthanide-activated phosphates such as Ba₂ P₄ O₇ :Eu and Ca₃(PO₄)_(2:) Ce. Further examples of phosphors are described in U.S. Pat.Nos. 4,100,101; 2,303,963; 3,163,610; 3,163,603 and 3,506,584 and in R.C. Pastor et al., Mat. Res. Bull., 15 469-475 (1980). Typicaltransparent phosphors include RbI:Tl; KI:Tl; BaFCl:Sr, Eu; BaSO₄ :Sr,Eu; CsCdF₃ Mn; BaF₂ ; KCdF₃ :Mn; and SrF₂. Preferred phosphors areRbI:Tl; KI:Tl; BaFCl:Sr, Eu; CsCdF₃ :Mn; BaSO₄ :Sr, Eu; and BaSO₄ :Pb.

The above-described phosphors are prepared by any conventional methodfor preparing isotropic phosphors, such as by introducing the anions andcations which form the phosphor into a reaction solution, maintaining anexcess of up to 1 molar of an anion or cation throughout the reactionmixture, preventing local excesses of cations or anions, and thus slowlygrowing crystals of the phosphor to at least 0.5 micron, as described inU.S. Pat. No. 3,668,142 issued June 6, 1972 to Luckey, which is herebyincorporated by reference. Other methods for preparing isotropicphosphors which are excited by x-rays and substantially transparent tothe emitted light, include precipitation at elevated temperatures andsuper-atmospheric pressures described in Ruthruff, U.S. Pat. No.2,285,464; precipitation followed by firing, fusion, and grinding to thedesired particle size; and ignition in the presence of a flux. Themethod of U.S. Pat. No. 3,668,142 is the preferred method for preparingthe isotropic phosphors.

These screens can be modified so that they are useful in the apparatusand methods for producing images that are described in U.S. Pat. No.3,859,527, German No. 2,951,501, and German No. 2,928,246. In thismodification an essentially isotropic storage phosphor is coated in abinder on a support that has the characteristics described below. Thephosphor is excited by a pattern of radiation of a first wavelength. Thephosphor is then exposed to radiation of a second wavelength whichcauses the said storage medium to emit a third wavelength of radiationhaving an intensity pattern representative of the stored image. Thebinder used in making this screen matches the index of refraction of thephosphor at the second wavelength and the support for the screen isselected so that it does not reflect the radiation at the secondwavelength. The index of refraction of the binder at the thirdwavelength is preferably selected so that it does not match that of thephosphor and the support of the screen may reflect the radiation at thethird wavelength. Thus, the radiation at the third wavelength, which isemitted when the phosphor is irradiated at the second wavelength, is nottrapped by total internal reflection or by the support, but escapes fromthe screen and is efficiently collected by a photomultiplier tube withappropriate optics or by other photosensors which respond efficiently tothe radiation at the third wavelength. Screens of this type areparticularly useful for radiography and other applications in which apattern of high energy radiation is absorbed by the phosphor, thenreleased by scanning the screen with a laser beam that has a wavelengthequal to that where the index of refraction of the phosphor and binderare matched and where the support of the screen has minimum reflectance.Ideally, the beam from the laser follows the path of the high energyradiation so that the resolution of the image from the screen isdetermined by the dimensions of the laser beam. The light released fromphosphor by the laser is collected by an appropriate photosensor,amplified, and the signal displayed on a cathode ray tube or recorded onan image recording medium to form the image. Appropriate phosphorscomprise the barium alkaline earth metal fluorohalides of German No.2,951,516, German No. 2,928,244, and other storage phosphors which haveindices of refraction less than about 1.75 in the visible region of thespectrum.

The phosphor crystals are optionally activated to obtain the desiredspeed by any conventional method of activation. One method is theaddition of a solution of a small amount (about 0.05 percent by weight)of the activating ion in a solvent, such as isopropanol, to a vigorouslystirred solution of the isotropic host in a solvent, such as water, atvery low temperatures (-30° to +20° C.), followed by collection of theprecipitated activated phosphor.

The substantially isotropic phosphors of the invention generally havecrystalline morphologies which are cubic or substantially cubic. Thesubstantially isotropic phosphors of the invention generally havecrystal sizes in the range from about 1 to about 50 microns, with thesize range from about 10 to about 20 microns being preferred.

The novel x-ray intensifying screen includes any polymer having an indexof refraction within 0.02 of the index of refraction of the phosphorover at least 80 percent of the emission spectrum.

The selection of the polymer for the novel x-ray intensifying screen isdependent on the index of refraction of the selected substantiallyisotropic phosphor at its emission wavelength. The index of refractionof the phosphor is determined by measuring the transmission spectra ofthe phosphor mixed with a series of Cargille liquids, as described in"The Particle Atlas", McCrone, Draftz and Delly, Ann Arbor SciencePublishers, Inc., 1967, and determining the wavelength at which theindex of refraction of the phosphor and the liquid match. A phosphordispersion curve is obtained by plotting the wavelengths of maximumtransmission for the series on the family of Cargille dispersion curvespublished in "The Particle Atlas" referred to above. The phosphordispersion curve thus obtained is used directly to find the index ofrefraction required for the polymer of the novel transparent x-rayintensifying screen.

The polymer having the required index of refraction, i.e., an index ofrefraction within 0.02 of the refraction of the phosphor over at least80 percent of its emission spectrum, comprises a single polymerizedmonomer, or the polymer comprises a mixture of two or more polymerizedcopolymerizable monomers. Generally, the polymer comprises twocopolymerizable polymerized monomers, one of which, when polymerized,provides a polymer of higher index of refraction than required, and onewhich, when polymerized, provides a lower index of refraction thanrequired. The relative proportions of the two monomers are adjusted toprovide the required refraction index. Calculated formulations areverified by measuring the transmission curve of a sample coating of thefluorescent composition of the novel intensifying screen on aspectrophotometer. A wavelength of maximum transmission which is lessthan that of the phosphor emission wavelength indicates that therefractive index of the polymeric binder is too low. A wavelength ofmaximum transmission which is greater than that of the phosphor emissionwavelength indicates that the refractive index of the polymer is toohigh.

Monomers which, when polymerized, provide an index of refraction higherthan that of the phosphor selected generally provide an index ofrefraction above 1.4, preferably in the range from 1.40 to 1.75.Examples of monomers which, when polymerized, provide an index ofrefraction higher than that of the phosphor selected, and thus can bemixed with monomers having a lower index of refraction to become usefulherein, include S-(1-naphthyl carbinyl) thioacrylate, naphthyl acrylate,1-bromo-2-napthylacrylate and naphthylmethacrylate. The preferredmonomer is S-(1-naphthyl carbinyl)thioacrylate.

Monomers which, when polymerized, provide an index of refraction lowerthan that of the phosphor generally provide an index of refractionranging from about 1.40 to about 1.75, preferably in the range from 1.40to 1.60. Examples of monomers which, when polymerized, provide an indexof refraction lower than that of the phosphor selected and thus areuseful when mixed with monomers having a higher index of refraction,include copolymerizable ethylenically unsaturated monomers such asacrylates and methacrylates such as methyl acrylate, ethyl acrylate,propyl acrylate, butyl acrylate, butyl methacrylate and cyclohexylmethacrylate; vinyl esters, amides, nitriles, ketones, halides, ethers,olefins, and diolefins as exemplified by acrylonitrile,methacrylonitrile, styrene, α-methyl styrene, acrylamide,methacrylamide, vinyl chloride, methyl vinyl ketone, fumaric, maleic anditaconic esters, 2-chloroethylvinyl ether, dimethylaminoethylmethacrylate, 2-hydroxyethyl methacrylate, N-vinylsuccinamide,N-vinylphthalimide, N-vinylpyrrolidone, butadiene and ethylene.Preferred monomers are acrylates and methacrylates, with cyclohexylmethacrylate being most preferred.

The proportion in which the above-described high-index and low-indexmonomers are mixed varies widely to provide a polymer having therequired index of refraction. The polymerized low-index monomerpreferably comprises from 5 to 100 mole percent of the resultingpolymer, with the range from 15 to 80 mole percent being most preferred.The polymerized high index monomer preferably comprises from 0 to 95mole percent of the resulting polymer, with the range from 20 to 85 molepercent being most preferred.

In one embodiment, the polymer of the novel intensifying screencomprises from 5 to 100 mole percent of recurring units having theformula: ##STR5## wherein: R¹ is H or alkyl, preferably containing fromabout 1 to about 4 carbon atoms, such as methyl, ethyl, propyl,isopropyl, and butyl; and

R² is alkyl, preferably containing from about 1 to about 12 carbonatoms, such as methyl, ethyl, propyl and butyl; cycloalkyl, such ascyclopentyl and cyclohexyl; aryl preferably containing from about 6 toabout 22 carbon atoms, such as phenyl, naphthyl, anthracene, perylene,acenaphthene and rubrene; aralkyl, preferably containing from about 5 toabout 20 carbon atoms, such as benzyl, phenylethyl, phenylpropyl,phenylbutyl, tolylbutyl and naphthylmethyl; or aryl substituted withalkyl, preferably containing from about 1 to about 20 carbon atoms, suchas methyl, ethyl, isopropyl and hexyl; alkoxy, preferably containingfrom about 1 to about 20 carbon atoms, such as methoxy and ethoxy; orheterocyclic, preferably a 5 to 7-membered ring which may be saturated,such as pyrrolidone, morpholine, piperidine, tetrahydrofurane, dioxaneand quinaldine, or unsaturated, such as pyrrole, isoxazole, imidazole,isothiazole, furazan and pyrazoline.

A preferred polymer of the novel x-ray intensifying screen furthercomprises from 0 to 95 mole percent of recurring units having theformula: ##STR6## wherein: Ar is arylene, preferably containing fromabout 6 to about 22 carbon atoms, such as phenylene, naphthalene,anthracene; perylene, acenaphthene and rubrene;

R¹ is H or alkyl as described for R² above;

R³ is H, alkyl, aryl, or aralkyl as described for R² above; and

R⁴ is H, alkyl, preferably containing from about 1 to about 20 carbonatoms, such as methyl, ethyl, isopropyl, and hexyl; alkoxy, preferablycontaining from about 1 to about 20 carbon atoms, such as methoxy andethoxy; amino; halogen such as chloride and bromide; sulfide; sulfoxide;sulfonate; or heterocyclic, preferably a 5 to 7-membered ring which maybe saturated, such as pyrrolidine, morpholine, piperidine,tetrahydrofurane, dioxane and quinaldine, or unsaturated, such aspyrrole, isoxazole, imidazole, isothiazole, furazan and pyrazoline.

It is noted that throughout the specification and claims the terms"alkyl", "aryl" and "arylene" include substituted alkyl, aryl andarylene, such as methoxy ethyl, chlorophenyl and bromonaphthyl.

Examples of polymers useful for the novel x-ray intensifying screeninclude:

poly[1-naphthyl carbinyl methacrylate-co-S-(1-naphthyl carbinyl)thioacrylate];

poly[1-naphthyl carbinyl methacrylate-co-1-bromo-2-naphthylacrylate];

poly[S-(1-naphthyl carbinyl) thioacrylate-co-benzyl methacrylate];

poly[S-(2-naphthyl carbinyl) thioacrylate-co-benzyl methacrylate]; and

poly[t-butyl methacrylate].

In an especially preferred embodiment, the polymer of the novelintensifying screen comprises from 5 to 100 mole percent of apolymerized co-polymerizable naphthyl carbinyl methacrylate monomer, andfrom 0 to 95 mole percent of a polymerized copolymerizable naphthylcarbinyl thioacrylate monomer. In a still further embodiment, thepolymer comprises from 5 to 100 mole percent of polymerized 1-naphthylcarbinyl methacrylate and from 0 to 95 mole percent of polymerizedS-(1-naphthyl carbinyl) thioacrylate.

The x-ray intensifying screen of the invention, comprising asubstantially isotropic phosphor, which is excited by x-rays andsubstantially transparent to light emitted by the phosphor, and apolymeric binder carefully selected so as to match, within 0.02, theindex of refraction of the phosphor, is highly transparent. Theintensifying screens of the invention generally exhibit a mean free pathfor light scatter greater than one millimeter, preferably greater than 3millimeters, for phosphor:binder ratios of 2.5 or larger. This highlytransparent screen material allows the use of relatively thick screenswhich absorb more of the incident x-ray beam and thus results in higherspeed. Further, the increased absorption of x-rays decreases quantummottle and allows improvement in overall image quality. Further still,the polymeric binder protects the fragile phosphors from physicaldamage.

The support for the x-ray intensifying screen of the invention includesany material having an index of refraction equal to or up to 0.05 unitshigher than the index of refraction of the phosphor of the invention,and having a reflection optical density of at least 1.7 to light emittedby the phosphor. Suitable materials include polymeric materials such asLucit® (poly(methyl methacrylate); Elbite (tourmaline); Formica®(poly(urea)formaldehyde resin); polyolefins such as polyethylene andpolypropylene; polycarbonates; cellulose acetate; cellulose acetatebutyrate; poly(ethylene terephthalate); glass such as Corning Fotoform®glass having 80 percent of its area covered with holes 0.015 inch deepand 0.005 inch in diameter; and metal such as black anodized aluminum.

The required reflection optical density of 1.7 to light emitted by thephosphor is provided by the use of support materials which areinherently darkly colored, materials which have been dyed or pigmentedduring manufacture to provide a uniform dark color throughout, ormaterials which have undergone a surface treatment such as coating witha dye, pigment or dyed or pigmented material, anodizing in the case ofmetals, or a combination of the above surface treatments.

The support of the invention also has an index of refraction equal to orup to 0.05 units higher than the index of refraction of the phosphor atits wavelength of maximum emission. In one embodiment, a preferredsupport having both the required optical density and the required indexof refraction comprises a conventional support material having a thinpolymeric layer on the surface on which the fluorescent composition isto be applied. This thin polymeric layer comprises a polymer having anindex of refraction equal to or up to 0.05 units higher than the indexof refraction of the phosphor at its wavelength of maximum emission, anda finely divided pigment such as carbon in an amount sufficient toproduce an optical density of 1.7 to light emitted by the phosphor.

The x-ray intensifying screen of the invention comprising a highlytransparent screen material having high speed and a light-absorbingsupport having the required reflection optical density, gives highcontrast and resolution. The use of a support which has the same or veryslightly higher (up to 0.05 higher) index of refraction as that of thephosphor layer decreases the flare of the image and increases contrast.

In another embodiment of the invention, a particularly advantageousfluorescent composition comprises:

(a) from 50 to 90 percent by weight of a substantially isotropicphosphor which is excited by x-rays and substantially transparent tolight emitted by said phosphor; and

(b) from 10 to 50 percent by weight of a polymer having an index ofrefraction within 0.02 of the index of refraction of said phosphor overat least 80 percent of the emission spectrum of said phosphor, saidpolymer comprising:

(i) from 5 to 99 mole percent of recurring units having the formula:##STR7## wherein: R¹ and R² are as described for the polymer of thenovel x-ray intensifying screen; and

(ii) from 1 to 95 mole percent of recurring units having the formula:##STR8## wherein: Ar, R¹, R³ and R⁴ are as described for the polymer ofthe novel x-ray intensifying screen.

Examples of polymers useful in the novel fluorescent compositioninclude:

poly[1-naphthyl carbinyl methacrylate-co-S-(1-naphthylcarbinyl)thioacrylate];

poly[S-(1-naphthyl carbinyl)thioacrylate-co-benzyl methacrylate]; and

poly[S-(2-naphthylcarbinyl)thioacrylate-co-benzyl methacrylate].

Preferred polymers which are useful in the novel fluorescent compositioninclude polymers comprising from 5 to 99 mole percent of a polymerizedco-polymerizable naphthyl carbinyl methacrylate monomer, and from 1 to95 mole percent of a polymerized copolymerizable naphthyl carbinylthioacrylate monomer. Especially preferred is a polymer comprising from5 to 99 mole percent of recurring units having the formula: ##STR9##from 1 to 95 mole percent of recurring units having the formula:##STR10##

The recurring units for the polymer and their relative proportions aregenerally selected to achieve the index of refraction previouslydescribed.

In a further embodiment of the invention, a process for making anintensifying screen comprises the steps of

(a) coating a mixture comprising:

(i) from 50 to 90 percent by weight of a substantially isotropicphosphor which is excited by x-rays and substantially transparent tolight emitted by said phosphor; and

(ii) from 10 to 50 percent by weight of at least one copolymerizablemonomer or mixture of monomers, said monomer or mixture of monomers,when polymerized, having an index of refraction within 0.02 of the indexof refraction of said phosphor over at least 80 percent of the emissionspectrum of said phosphor,

on a support having an index of refraction equal to or up to 0.05 unitshigher than the index of refraction of said phosphor and having areflection optical density of at least 1.7 to light emitted by saidphosphor; and

(b) polymerizing said mixture coated on said support to produce apolymer comprising recurring units of said monomer or monomer mixture.

The mixture comprising the fluorescent composition of the novelintensifying screen is preferably prepared by combining a substantiallyisotropic phosphor in the form of a free-flowing powder with apolymerizable monomer or mixture of copolymerizable monomers which, whenpolymerized, exhibit the required index of refraction. The usefulphosphor to monomer ratio varies widely, but preferable ranges are from50:50 to 90:10 by weight, and more preferably in the range from 70:30 to80:20 by weight. Generally, the phosphor to monomer ratio is maximized,resulting in a honey-like, viscous mixture, which is capable of beingpoured. The resulting mixture is optionally degassed to remove trappedair bubbles.

The mixture optionally further comprises from 0.001 to 1.0 percent byweight, preferably from 0.1 to 0.5 percent by weight of a photoinitiatorsuch as 4,4'-bis-chloromethyl benzophenone, benzoin methyl ether, andbenzoyl peroxide. It is noted that further additional components areoptionally included in the mixtures of the novel process. For example,resins, stabilizers, surface active agents and mold release agents serveto improve film formation, coating properties, adhesion of the mixtureto the support, separability of the mixture from non-support materials,mechanical strength and chemical resistance.

The mixture of the novel process is coated onto the support to apre-determined thickness by techniques well-known in the art, such asroll coating, brush coating, solvent coating or x-hopper coating. Onemethod of coating the mixture comprises pouring the mixture onto thedesired support, covering it with a cover sheet, such as a glass coversheet, having appropriate spacers to produce a predetermined coatingthickness, and spreading the mixture by applying pressure to the coversheet to the limit of the spacers.

The optimum coating thickness of the phosphor-monomer mixture dependsupon such factors as the use to which the coating will be put, the speeddesired, the degree of image quality desired, the phosphor selected, themonomer or monomer mixture employed, the phosphor to monomer ratio andthe nature of other components which may be present in the coating.Useful coating thicknesses for use in preparing x-ray intensifyingscreens are from 25 to 2500 microns, with coating thicknesses of from400 to 1200 microns being preferred. The preferred coating coveragelikewise varies widely between about 10 g and about 500 g/ft², with therange from 50 to 200 g/ft² being preferred.

The coating, comprising a monomer or mixture of monomers and a phosphor,is preferably polymerized at a temperature of 20°-30° C. by irradiationwith a near-ultraviolet lamp. Other methods of polymerization aresimilarly used. Such methods include thermal polymerization,polymerization by election beam radiation and polymerization by highenergy gamma irradiation.

After polymerization, the polymerized mixture is preferably cooled toroom temperature or below, and any cover sheet used to spread the coatedmixture and establish coating thickness is removed. In some cases,release is gently initiated, by inserting a blade between the supportand the cover sheet to separate the support from the coated polymerizedmixture, until Newton's rings are observed at the initiation site. Thecover sheet is then lifted away, optionally further cooling the coversheet briefly, for example, with powdered dry ice. Further coolingshould be carefully undertaken, however, as overcooling the cover sheetis likely to shatter the polymerized, coated screen mixture.

The resulting polymer has an index of refraction within 0.02 of theindex of refraction of the phosphor over 80 percent of its emissionspectrum, thus maintaining a high degree of transparency to the lightemitted by the excited phosphor. The polymer protects the phosphor frommechanical damage, and, if hydrophobic, from damage caused by moisture.

The process of the invention thus provides a highly transparent x-rayintensifying screen having satisfactory speed, high contrast and highresolution. Further the process as described provides a relativelyinexpensive and straightforward method of manufacturing high speed, highresolution x-ray intensifying screens without the addition of reflectingpigments.

The following preparations and examples are included for a furtherunderstanding of the invention.

PREPARATION 1

The phosphor RbI:Tl (0.0004) was prepared by adding a solution of 0.33 gof thallous acetate in 500 ml of isopropanol at a rate of 36 ml/min to avigorously stirred solution of 636 g rubidium iodide in 460 g of water.The temperature of the isopropanol solution was maintained at -29° C.,and the temperature of the aqueous solution was maintained at about 15°C. 200 g of the precipitated rubidium iodide phosphor was collected,carefully removing all of the supernatant isopropanol water mixture,which was reserved for recovery of unprecipitated rubidium iodide to beused in subsequent preparations. (Any supernatant isopropanol-watermixture remaining with the precipitated phosphor can contaminate theprecipitated phosphor with further precipitation of a phosphor differingin composition, and cause unwanted scattering of light in the resultingfluorescent composition.) The precipitated thallium-activated RbIphosphor, being free of supernatant isopropanol-water mixture, was thenwashed twice with isopropanol in a high speed, food-processing blender,and the precipitate collected on glass filter paper after each washing.The phosphor was vacuum dried and bottled. The speed of the RbI:Tl(0.0004) thus prepared was about equal to that of KI:Tl, and speedsbetween 6 and 7 times greater than that of DuPont No. 501 commercialCaWO₄ phosphor were obtained in the x-ray powder test described in U.S.Pat. No. 3,668,142, previously referred to herein.

PREPARATION 2

The phosphor KI:Tl (0.0003) was prepared by adding a solution of 0.4 gthallous acetate in 1.6 liters of isopropanol at -29° C. to a solutionof 800 g potassium iodide in 600 g distilled water at 15° C. withvigorous stirring. The temperature of the supernatant solution wasmaintained at about 14° C. The rate of addition was 35 ml/min. Thecrystals of the precipitated phosphor were free from defects and hadcubic morphology with crystal sizes in the range from about 10-20microns. The speed of the phosphor, measured after precipitation,washing, and drying, by the method used in U.S. Pat. No. 3,668,142 wasabout seven times that of commercial calcium tungstate.

PREPARATION 3

A mixture of 66 g of cyclopentadiene and 500 ml of methylene chloridewas stirred with 90 g of acryloyl chloride at dry ice temperature(-78.5° C.) and allowed to warm slowly to room temperature over 24hours. The reaction product was then distilled. The resultingbicycloheptane carbonyl chloride thus obtained was allowed to react with1-(naphthylcarbinyl)mercaptan and refluxed in methylene chloride (b.p.40°-41° C.) while one equivalent of diisopropylethylamine was slowlyadded to the mixture. The product was vacuum distilled, using a 250° C.oil bath, under which conditions the cyclopentadiene split off, givingS-(1-naphthylcarbinyl)thioacrylate in good yield. A thin-layerchromatograph (50:50 hexane/ether, silica gel) of the resulting productindicated an R_(f) value of 0.69 to 0.72.

PREPARATION 4

1-naphthyl carbinyl methacrylate was prepared by catalytictransesterification of an excess quantity of methyl methacrylate withthe alcohol 1-naphthyl carbinol. The by-product, methanol, wascontinuously removed by azeotropic distillation and/or use of molecularsieves, thus pulling the reversible reaction towards completion. Whenthe reaction was essentially complete, the excess methyl methacrylatewas removed by distillation at atmospheric pressure. A small amount(from 5 to 25%) of the unreacted higher alcohol 1-naphthyl carbinolremained in the resulting 1-naphthyl carbinyl methacrylate.

PREPARATION 5

In an alternative synthesis of 1-naphthyl carbinyl methacrylate,1-(chloromethyl)naphthalene is treated with one equivalent of potassiummethacrylate in dimethyl sulfoxide. The potassium methacrylate employedis either previously isolated or formed in situ from potassium hydroxideand methacrylic acid. The reaction is continued at 70° C. for 30minutes. The resulting 1-naphthylcarbinyl methacrylate is isolated in93-98% yield, virtually free from contaminants.

PREPARATION 6

Aluminum plates were anodized in 12-15 percent H₂ SO₄ at 70° and 12-14amperes/ft². The porous deposit was treated with aluminum Black BK® dye(a registered trademark of Sandoz Colors and Chemicals) and then sealedwith hot water or nickel acetate solution. The resulting supportexhibited an optical density of 2.34 when overcoated with a mixture ofrubidium iodide and polymer having matched indexes of refraction.Although the index of refraction of anodized aluminum is not preciselyknown, it is thought to be about 1.76, which is less than 0.05 unitshigher than that of rubidium iodide at 425 nm, the region of maximumemission.

EXAMPLE 1

A mixture of 100 g of thallium-activated potassium iodide phosphor(0.0003), as prepared in Preparation 2, and 40 g of a 4:1 mixture ofS-(1-naphthyl carbinyl) thioacrylate, as prepared in Preparation 3, and1-naphthyl carbinyl methacrylate, as prepared in Preparation 4,containing 0.3 percent by weight of 4,4'-bis-chloromethyl benzoquinonewas degassed under vacuum. A portion of the mixture was photopolymerizedbetween two glass sheets to form an unsupported screen, and released.The unsupported screen was placed in a Cary® 17 spectrophotometer andits optical density was measured using an unsupported screen containingonly photopolymerized polymer (lacking the phosphor) as a reference. Theoptical density of the unsupported screen was used to oalculate the meanfree path of light through the screen. The mean free path was calculatedto be at least 2.3 mm.

Another portion of the mixture was coated at different thicknesses on ablack anodized aluminum support as prepared in Preparation 6, andphotopolymerized under glass cover sheets. Radiographs were made byexposing Lo-Dose® film in contact with these experimental supportedscreens as back screens with 70 kVp x-rays. A control radiograph wasmade by likewise exposing film Lo-Dose® film in contact with a DuPont®Par Speed Intensifying screen in order to obtain the relative speeds ofthe experimental screens. The difference in speed was calculated throughthe known density vs. log exposure curve for Lo-Dose® film from thedensities which resulted on the exposed and developed films. Thefollowing results were obtained.

    ______________________________________    Screen  Screen      Relative   10 micron lead    Thickness            Coverage    Speed      bar test object    (microns)            (g/ft.sup.2)                        PAR = 100* Resolution    ______________________________________    405      61         175        3.15 lp/mm    750     113         265        2.24-2.5    1115    168         325        2.0    ______________________________________     *DuPont ® Par Speed Intensifying Screen

EXAMPLE 2

A mixture of 35.5 g of the thallium-activated rubidium iodide phosphor(0.0004) as prepared in Preparation 1 and 10 g of a 60:40 mixture of1-naphthyl carbinyl methacrylate and 1-bromo-2-naphthylacrylatecontaining 0.3 percent 4,4'-bis-chloromethyl benzophenone was spread ona black anodized aluminum support and covered with a glass sheet whilebeing photopolymerized. When polymerization was complete, the glasssheet was released. The resulting transparent screen was 500 micronsthick and had a coverage of 89 g of phosphor per square foot.Radiographs made with this screen as a back screen with Lo-Dose® Film at70 kVp gave a relative radiographic speed (calculated as in Example 1)of 255 compared to 285 for a DuPont Hi-Plus® Screen with Lo-Dose® Film.When a bone and bead test object was employed in the same comparison,better image quality was obtained with the transparent screen.

EXAMPLE 3

A mixture of 250 g of the thallium-activated rubidium iodide and 65 g ofa 3:1 mixture of 1-naphthyl carbinyl methacrylate and S-(1-naphthylcarbinyl) thioacrylate which also contained 0.3 percent by weight of4,4'-bis-chloromethyl benzophenone was degassed under vacuum and thencoated three ways: (1) on black anodized aluminum support, (2) onreflective aluminum support on an optically flat surface, and (3) on nosupport (self-supporting film). All three coatings were of equalthickness and were photopolymerized. Radiographs were made with thesethree screens, along with the DuPont Hi-Plus® screen, using Lo-Dose®Film, 70 kVp x-rays and a 20 μ lead bar test object. The resolution ofthe radiographs was as follows:

    ______________________________________    Hi-Plus ® Screen                       4.0 lp/mm    Black Aluminum Support                       4.0 lp/mm    Reflective Aluminum                       1.8 lp/mm    Support    Unsupported        1.8 lp/mm    ______________________________________

The resolution of the screen having a black support showed a dramaticincrease both over the resolution of the screen having a reflectivesupport and over that of the unsupported screen.

EXAMPLE 4

A mixture of 136.8 g of thallium-activated rubidium iodide (0.0004),40.0 g of a 3:1 mixture of 1-naphthyl carbinyl methacrylate containingup to 25 percent 1-naphthyl carbinol and S-(1-naphthyl carbinyl)thioacrylate, and 0.3 percent by weight of 4,4'-bis-chloromethylbenzophenone was degassed under vacuum. The mixture was then coated on asupport consisting of inlaid strips of black polished Formica®, blackanodized aluminum, black Corning Fotoform® glass having 80 percent ofits area covered with holes, 0.005 inch in diameter and 0.015 inch deepand dark blue tourmaline in a matrix of black Lucite® plastic. Themixture was spread evenly across the support so that the different typesof support were coated with an equal thickness of the mixture. A glasscover sheet was placed on the mixture, and the mixture wasphotopolymerized. The cover sheet was removed, and the reflectionoptical densities of the different areas were measured. A 70 kVpradiograph of a 10 micron lead bar resolution test object was made thescreen as a back screen with DuPont Lo-Dose® film. The radiograph madeusing this transparent screen was compared with a control radio madewith Lo-Dose® film and using an opaque Hi-Plus® screen. Radiographicspeed was determined as in Example 1. The results obtained were asfollows:

    ______________________________________                         Reflection                                   Radio-               Refractive                         Optical   graphic                                         Resolution    Support    Index n.sub.d                         Density   Speed (lp/mn)    ______________________________________    Lucite ®               1.49      2.25      315   2.5-2.8    Fotoform ®               --        1.87      250   3.15    Glass    tourmaline 1.64      2.57      250   3.15    Formica ®               1.65      2.17      245   3.15-3.55    Black anodized               1.76      2.34      245   3.55    Aluminum    Hi-Plus ® Screen               --        --        285   3.55    Control (opaque)    ______________________________________

The results indicated that the optimum combination of speed andresolution were obtained when the fluorescent composition mixture wascoated on a black anodized aluminum surface for the particulartransparent phosphor-polymer combination selected. Further, the resultsshowed that the transparent screen having a black anodized aluminumsupport exhibited resolution equal to and radiographic speed nearlyequal to the conventional opaque control screen; however, thetransparent screen of the invention displayed less quantum mottle thanthe conventional opaque screen.

EXAMPLE 5

A mixture of 180 g of finely powdered Ba₀.94 Sr₀.06 ECl:Eu (0.006)phosphor and 51 g of a blend of benzyl methacrylate and1-naphthylcarbinyl methacrylate (approximately 50:50 by weight) wasdegassed under vacuum and spread on a black anodized aluminum support. Aglass cover sheet was placed on top of the layer and the mixture waspolymerized by irradiation with an ultraviolet lamp with substantialemission at 365 nm through the glass. After the glass was removed, thearea of the layer and the weight were recorded. Coverage of the screenwas calculated as 85 g/ft² of phosphor. The mean free path for 380 nmradiation was measured spectrophotometrically as 304 microns.

The screen was used as a back screen with a 58 g/ft² Gd₂ O₂ S:Tb (on ahighly reflecting support) front screen to make 70 kVp radiographs of astandard "bone and bead" test object with KODAK X-OAT R x-ray film. Thecontrol for the image quality evaluations was made with both front andback Gd₂ O₂ S:Tb screens. The speed of the control was 400 and theresolution was 2.24 1 lp/mm. The transparent back screen gave a speed of350 and a resolution of 2.24 1 lp/mm. The mottle of both radiographs wasjudged about equal, but the sharpness and bead visibility were superiorin the transparent screen radiograph.

The invention has been described in detail with particular reference tocertain embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

What is claimed is:
 1. A fluorescent coating composition comprising:(a)from 50 to 90 percent by weight of a substantially isotropic phosphorwhich is excited by x-rays and substantially transparent to lightemitted by said phosphor; and (b) from 10 to 50 percent by weight of apolymer having an index of refraction within 0.02 of the index ofrefraction of said phosphor over at least 80 percent of the emissionspectrum of said phosphor, said polymer comprising:(i) from 5 to 99 molepercent of a polymerized co-polymerizable naphthyl carbinyl methacrylatemonomer; and (ii) from 1 to 95 mole percent of a polymerizedco-polymerizable naphthyl carbinyl thioacrylate monomer.
 2. Thefluorescent composition of claim 1 wherein said phosphor is an alkalimetal compound.
 3. The fluorescent composition of claim 1 wherein saidphosphor is selected from the group consisting of RbI:Tl and KI:Tl.
 4. Afluorescent composition comprising:(a) from 50 to 90 percent by weightof a substantially isotropic alkali metal phosphor which is excited byx-rays and substantially transparent to light emitted by said phosphor;and (b) from 10 to 50 percent by weight of a polymer having an index ofrefraction within 0.02 of the index of refraction of said phosphor overat least 80 percent of the emission spectrum of said phosphor, saidpolymer comprising:(i) from 5 to 99 mole percent of recurring unitshaving the formula: ##STR11## (ii) from 1 to 95 mole percent ofrecurring units having the formula: ##STR12##
 5. The fluorescentcomposition of claim 3 wherein said phosphor is selected from the groupconsisting of RbI:Tl and KI:Tl.
 6. An x-ray intensifying screencomprising a support having thereon a fluorescent compositioncomprising:(a) from 50 to 90 percent by weight of a substantiallyisotropic phosphor which is excited by x-rays and substantiallytransparent to light emitted by said phosphor; and (b) from 10 to 50percent by weight of a polymer having an index of refraction within 0.02of the index of refraction of said phosphor over at least 80 percent ofthe emission spectrum of the phosphor, said polymer comprising:(i) from5 to 100 mole percent of polymerized co-polymerizable naphthyl carbinylmethacrylate monomer; and (ii) from 0 to 95 mole percent of apolymerized co-polymerizable naphthyl carbinyl thioacrylate monomer;said support having an index of refraction equal to or up to 0.05 unitshigher than the index of refraction of said phosphor and having areflection optical density of at least 1.7 to light emitted by saidphosphor.
 7. The x-ray intensifying screen of claim 6 wherein saidphosphor is an alkali metal compound.
 8. The x-ray intensifying screenof claim 7 wherein said phosphor is selected from the group consistingof RbI:Tl and KI:Tl.
 9. The x-ray intensifying screen of claim 6 whereinsaid support comprises a black anodized aluminum surface.
 10. An x-rayintensifying screen comprising a support having thereon a fluorescentcomposition comprising:(a) from 50 to 90 percent by weight of asubstantially isotropic alkali metal phosphor which is excited by x-raysand substantially transparent to light emitted by said phosphor; and (b)from 10 to 50 percent by weight of a polymer having an index ofrefraction without 0.02 of the index of refraction of said phosphor overat least 80 percent of the emission spectrum of said phosphor, saidpolymer comprising:(i) from 5 to 100 mole percent of a polymerizedcopolymerizable naphthyl carbinyl methacrylate monomer; and (ii) from 0to 95 mole percent of a polymerized copolymerizable naphthyl carbinylthioacrylate monomer; said support comprising a black anodized aluminumsurface having an index of refraction equal to or up to 0.05 unitshigher than the index of refraction of said phosphor and having areflection optical density of at least 1.7 to light emitted by saidphosphor.
 11. The x-ray intensifying screen of claim 10 wherein saidphosphor is selected from the group consisting of RbI:Tl and KI:Tl. 12.The x-ray intensifying screen of claim 10 wherein said naphthyl carbinylmethacrylate monomer is 1-naphthyl carbinyl methacrylate and whereinsaid naphthyl carbinyl thioacrylate monomer is S-(1-naphthylcarbinyl)thioacrylate.
 13. An x-ray intensifying screen comprising asupport havihg thereon a fluorescent composition comprising:(a) from 50to 90 percent by weight of a substantially isotropic alkali metalphosphor which is excited by x-rays and substantially transparent tolight emitted by said phosphor, selected from the group consisting ofRbI:Tl and KI:Tl; and (b) from 10 to 50 percent by weight of a polymerhaving an index of refraction within 0.02 of the index of refraction ofsaid phosphor over at least 80 percent of the emission spectrum of saidphosphor, said polymer comprising:(i) from 5 to 100 mole percent ofpolymerized 1-naphthyl carbinyl methacrylate monomer; and (ii) from 0 to95 mole percent of polymerized S-(1-naphthyl carbinyl) thioacrylatemonomer; said support comprising a black anodized aluminum surfacehaving an index of refraction equal to or up to 0.05 units higher thanthe index of refraction of said phosphor and having a reflection opticaldensity of at least 1.7 to light emitted by said phosphor.